Power control and soft handover are two crucial ingredients of the Wideband Code Division Multiple Access (WCDMA) air interface.
Power control is used to continuously adjust the transmitted power such that the received quality is good enough, but not better. This way, not more than necessary interference is created. The principle of the power control is that the receiver sends power control commands, “Transmit Power Control (TPC) bits”, each indicating either “down” or “up”, depending on whether it's received quality is good enough or not.
Soft handover provides a gain in fast fading radio environments, often referred to as “macro diversity”. The user equipment is then in communication with two or more base stations at the same time. The overall quality is good enough if at least one of the radio links has good enough quality.
The power control ensures that this is the case. Each base station transmits power control commands depending on its received quality. The user equipment combines the power control commands and increases its power only if all base stations request a power increase, otherwise it decreases its power.
On the uplink (UL), each base station receives and tries to decode the data sent by the user equipment. The result of this attempt is then passed along to a diversity-combining unit (located in the radio network controller, RNC), in which data from all involved base stations is received. The diversity-combining unit considers the data received from all involved base stations, and selects the one that is most likely to be correct.
Note that any particular base station is not in general capable of correctly decoding all, or even most of, the data transmitted by the user equipment. This is because the user equipment's transmitted power will be set such that only the currently most favourable base station will have good enough quality.
In an uplink from a user equipment to a radio base station, the transmission powers of different uplink channels usually are set relative to a reference channel, for example in Wideband Code Division Multiple Access (WCDMA) the Dedicated Physical Control Channel (DPCCH). The power difference relative to the reference channel is called power offset, i.e. a transmission power value on the uplink channel is determined by adding a power offset value to a pre-determined reference power value.
While most of the data transmitted by the user equipment is directed towards one or more destinations in the network, some data is intended to be used by the base stations directly. In the Third Generation Partnership Project (3GPP) Rel. 99, such data is part of the uplink dedicated physical control channel, or uplink DPCCH. Examples of such data are the transmit power control (TPC) field used for downlink power control and the feedback (FB) field. In 3GPP Rel. 5, an additional uplink physical control channel, the HS-DPCCH, with more data fields of this type is introduced, in particular an acknowledgement field for the high-speed downlink shared channel (HS-DSCH). Some of these data fields are intended to be used by all involved base stations, while other fields are to be used by one particular base station.
For UL DPCCH, a problem with this type of data occurs in soft handover, when a particular base station may be unable to decode the data that it needs. This may cause performance degradation of vital functions in the base station, such as downlink power control or automatic retransmissions.
For UL DPCCH, a traditional solution to the mentioned problem is to use a fixed higher power (or, equivalently, repeated bits) for the data fields that are to be decoded by all involved base stations. This traditional solution has, however, a drawback of higher power consumption and a larger interference, even in situations where the extra power is unnecessary.
For UL HS-DPCCH, a problem is that when the UL power is controlled by the non-HS serving cell, HS-DPCCH transmission power decreases (because the user equipment is approaching to that non-HS cell). As a result, the transmitted HS-DPCCH power is not enough to overcome its UL pathloss back to the HS serving cell and the HS-DPCCH performance becomes very bad. The base station can not decode correct information (e.g. wrong channel quality indication (CQI) or Hybrid Automatic Repeat Request (HARQ) acknowledgement (ACK)) from HS-DPCCH and uses this incorrect information to schedule the transmission. Finally, the HSDPA performance could be impacted.
An example of an unbalanced UL/DL problem explanation is shown in FIG. 2, in which the UE is moving from cell A (serving cell) served by RBS 15a to cell B (non-serving cell) served by RBS 15b illustrated with arrow M. The diagrams comprise a soft handover area, SHO. The Ec/No of cell A is denoted 21 and the Ec/No of cell B is denoted 22 in the upper diagram of FIG. 2. In the lower diagram of FIG. 2, cell A's pathloss+IUL is denoted 23 and cell B's pathloss+IUL is denoted 24. The UE follows the continuous line 25 to control its Tx power. As can be seen from FIG. 2, in the area 26, cell A is serving cell but the uplink power is controlled by cell B, which will cause a bad HS-DPCCH quality received by the serving cell A, and could have a wrong detection of CQI or HARQ acknowledgement.
Similar to the case for the DPCCH, a traditional solution for HS-DPCCH is either to use different power offsets for soft handover (SHO) users and non-SHO users or to use an ACK/NACK repetition method to improve the quality. Field test results have, however, shown that a larger HS-SCCH power offset for SHO users doesn't improve the HS-DPCCH quality so much in an unbalanced cell case.
An efficient power setting of HARQ feedback channels has been proposed where errors in the feed back information for HARQ processes are detected and this information is used to adjust the power for the feedback channel. This method, however, relies only on the ACK/NACK transmission and will only react when the power is already too low
Thus there is a need for an improved power offset control in soft handover.